Bash: Case Statements

A case statement compares a value with several patterns and runs the commands for the first matching pattern.

Basic Case Statement

Choosing a command

case "$1" in
    start)
        echo "Starting the service"
        ;;
    stop)
        echo "Stopping the service"
        ;;
    *)
        echo "Usage: $0 {start|stop}"
        ;;
esac

Each pattern is followed by ). End a branch with ;;, and close the statement with esac. The * branch is a fallback for values that do not match another pattern.

Matching Several Values

Combining patterns

case "$1" in
    yes|y|Y)
        echo "Continuing"
        ;;
    no|n|N)
        echo "Stopping"
        ;;
    *)
        echo "Please answer yes or no"
        ;;
esac

Separate alternative patterns with | when they should run the same block of commands.

Wildcard Patterns

Matching file names

case "$1" in
    *.sh)
        echo "Bash script"
        ;;
    *.txt|*.md)
        echo "Text document"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

Case patterns use shell wildcards: * matches any number of characters, ? matches one character, and bracket expressions such as [[:digit:]] match a character class.

Case Patterns for Validation

Checking a positive integer

    case "$1" in
    ''|*[!0-9]*)
        echo "Enter a positive integer"
        ;;
    *)
        echo "Valid integer: $1"
        ;;
esac

The first pattern matches an empty value or any value containing a non-digit. The second pattern is reached only when every character is a digit.

Regular Expressions

Use `=~` for regex matching

if [[ "$1" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; then
    echo "Date format is valid"
else
    echo "Use YYYY-MM-DD"
fi

Bash case patterns use shell wildcard syntax, not regular expressions. For a true regular-expression check, use [[ string =~ regex ]]. Keep the regex unquoted so Bash can interpret it as a pattern.

Reading Input in a Loop

Simple menu

while true; do
    read -r -p "Choose start, status, or quit: " command

    case "$command" in
        start)
            echo "Starting"
            ;;
        status)
            echo "Running"
            ;;
        quit|exit)
            break
            ;;
        *)
            echo "Unknown command"
            ;;
    esac
done

A case statement is useful for command-line menus because each supported command gets a readable branch and the fallback handles unexpected input.